Wavelength Division Multiplexing (WDM) techniques have been utilized to significantly enhance the signal capacity of optical communication systems. WDM systems simultaneously transmit multiple information signals on a single waveguide medium at different wavelengths or channels. Examples of such communication systems include, telecommunications systems, cable television systems, local area networks (LANs) and wide area networks (WANs). In a WDM system, optical signals are generated and multiplexed onto a plurality of optical channels, transmitted over a single optical waveguide, and demultiplexed at one or more destination terminals. Dense WDM (DWDM) systems are characterized by relatively close spacings between the respective channels.
WDM or DWDM communication systems may carry signals over many miles, with the system having a number of different origination and destination terminals or nodes. In many of these systems, channels are added/dropped from the WDM signal corresponding to one or more different origination/destination nodes. This form of optical signal routing is generally referred to as "add/drop multiplexing."A number of different devices and configurations have been employed as add/drop multiplexers. One approach is explored in Giles and Mizrahi, "Low-Loss ADD/DROP Multiplexers for WDM Lightwave Networks," IOOC Technical Digest, (The Chinese University Press, Hong Kong) c. 1996, pp. 65-67, the disclosure of which is incorporated herein by reference. In this paper, an add-drop multiplexer is proposed which uses two three-port optical circulators with a fiber grating positioned therebetween. Using this configuration, an optical signal to be dropped from multiplexed optical signals is reflected by the fiber grating and exits through a drop port of the first optical circulator. All other input signals exit via a through port of the first optical circulator. Similarly, an optical signal to be added which has a wavelength nominally identical to the optical signal being dropped from the optical transmission path is input to an add port of the second circulator. The signal to be added to the optical transmission path is reflected towards a through port of the second circulator by the same fiber grating disposed between the first and second circulators used for signal dropping.
A disadvantage associated with this type of add/drop multiplexer is the loss associated with the grating and, more significantly, the loss contributed by each of the two circulators. These loss values have a negative effect on the overall system loss budget, i.e. the total amount of optical loss that a given optical link can tolerate while maintaining signal integrity. Moreover, this loss may accumulate over a plurality of nodes each including one or more add/drop multiplexers.
Accordingly, it would be advantageous to provide an optical add/drop multiplexer with reduced optical loss adaptable for use with an optical communication system employing wavelength division multiplexing. Other and further objectives will be apparent from the following detailed description and the appended claims.